The present invention relates to a joining assembly including a support member of plastic material and a threaded element of plastic material. The threaded element is threaded into a receiving bore of the support member and comprises a core and an external thread which when threaded into the receiving bore of the support member forms a counter-thread in the receiving bore of the support member. Furthermore, the present invention relates to a threaded element formed as a screw or a threaded sleeve for such a joining assembly, and a tool and method for making a joining assembly.
Such joining assemblies comprising a plastic support member and a plastic threaded element formed as a screw or a threaded sleeve with a self-tapping external thread have become known, see for example DE 42 27 272 A1, DE 42 27 274 C2, and DE 23 17 736. The threaded elements of these joining assemblies generally are provided with a specifically designed thread profile so as to enable them to perform their self-tapping function.
For example DE 42 27 272 A1 discloses a threaded element with a so-called round profile, wherein the thread in an axial cross-section is of a circular profile, the revolutions of the thread being spaced from each other, in an axial direction, by small gaps. DE 42 27 274 C2 discloses a threaded sleeve having an external thread of a height which increases from a minimal value to a maximal value in the threading direction so as to provide, at the maximal value of the thread height, a step intended to act as a disengagement lock. The thread is formed as a “triangular thread” in this case.
DE 32 01 846 A1 and DE 91 15 162 U1 disclose self-tapping threaded elements having threads of a profile specially designed to perform the self-tapping function. However, these threaded elements are not made of plastic material.
DE 23 17 736 discloses a self-tapping screw or bolt of plastic material which includes longitudinally extending grooves for providing a pair of diametrically opposite cutting edges in each thread revolution, which cutting edges act in the threading operation to cut an internal thread into the receiving bore of the support member when the screw or bolt is threaded into the receiving bore.
While self-tapping threaded elements of plastic material are being used in practice, further improvements are desirable. One drawback of the prior art threaded elements of plastic material is that they are generally suited only for support members of one specific plastic material while they are not suited for a great number of other plastic materials. Quite often the self-tapping qualities of the threaded element are poor. The strength and load capability of the threaded element and of the joining assembly may be detrimentally affected by notching and/or clamping effects during forming of the thread. Furthermore, deformation resistance and stability of the threaded element are not always satisfactory. Finally, even for small threading torques maximal security against inadvertent disengagement of the threaded element from the receiving bore of the support member should be ensured.